I am a total layman when it comes to aviation but for a school presentation I am trying to understand the reasons why Air Canada 624 crashed while trying to land on runway 05 at Halifax (CYHZ). I have read the investigation report and the press release the Air Canada pilots union have published. The press release says:

ACPA strongly advocates for all major airports to have the necessary equipment to offer both lateral and vertical guidance and high intensity lighting to allow pilots to orient aircraft for landing.

Many runways at Canada’s largest airports lack the ground-based navigation systems which provide vertical guidance – that is, the distance between the aircraft and the ground – to orient an aircraft for landing.

I was under the impression that an altitude reading is always available through the radio altimeter or the barometric altimeter.

Why would such a "ground-based navigation system" that provides "distance between the aircraft and the ground" be needed? What can it provide that the regular altimeters on board cannot provide?

  • $\begingroup$ If the altimeter was used to approach the runway, the required altitude would need to be updated as the distance to the runway changes, this can only be done by a computer. The advantage of the ground guidance is that it doesn't need any calculation, or even to know the distance to the runway. It can be a light beam or a radio beam transmitted from the ground and the aircraft just follows it. $\endgroup$
    – mins
    Commented Jun 7, 2017 at 22:13
  • $\begingroup$ Thank you for your answer. I did not know that an ILS sends a beam that the aircraft can follow, this is new information to me. About the need to calculate the required altitude as a function of distance, can't this be calculated beforehand and tabulated? It would solve the problem without needing expensive ground guidance systems, wouldn't it? $\endgroup$ Commented Jun 8, 2017 at 11:52
  • $\begingroup$ It could be pre-calculated. This would still be an additional workload and a distance measuring equipment would be required (on the ground and onboard). The information could also be displayed on the altimeter, but I believe at the time, displaying this additional information was too difficult. Ref the ILS beams (loc and glide), each beam is actually a pair of beams and the receiver in the aircraft senses the axis of symmetry, where the beams are received with an equal intensity. $\endgroup$
    – mins
    Commented Jun 8, 2017 at 17:29

1 Answer 1


They mean an ILS - Instrument Landing System.

An ILS consist of:

  • A localizer component, which tells the angle between the aircraft's position and the extended runway centerline. In short, it tells the pilot whether they are left or right.
  • A glideslope component, which tells the angle between the aircraft's position and the extended runway surface on the ground. Typical glide slope angle is 3 degrees. In short, it tells the pilot whether they are high or low.

An ILS is usually coupled with an DME (Distance Measure Equipment), which tells the distance between the aircraft and the runway threshold. Using the glideslope angle together with the horizontal distance, one can compute the height above runway level.

From page 12 of the report:

AC624 was then cleared for the straight-in LOC approach ...

Which means only the lateral (left or right) component is available, there is no signal which tells the pilot whether they are high or low relative to the ideal descent path.

A radio altimeter is not very useful when the approach path leading to the runway has uneven terrain. It tells the height above the ground directly under the aircraft.

A barometric altimeter needs to be calibrated by a reading at ground level. It tells the height above sea level.

Both of these altimeters cannot tell the height above runway level. Therefore the pilots must rely on other data to monitor the descent profile, which increases the pilots' workload. In a situation such as low visibility and high crosswinds, this presents a very challenging situation.

If both vertical and horizontal guidance is available, then the pilots can simply follow this guidance all the way to the runway. They can also program the autopilot to follow this guidance, if suitable equipment is installed on the ground as well as onboard the aircraft.

enter image description here

Note the green bars: one is horizontal and one is vertical. They form a cross on the display. If the horizontal bar is up a little bit, the pilot would pull up a bit. If the vertical bar is to the right, the pilot would turn right a bit. These green bars only possible when ILS is used.

It is possible to compute the relative height by using a DME and a barometric altimeter only. For example, I am now 5.5 nautical miles from the runway threshold, and according to the approach chart, I should be at 1,800 feet. Right now my altimeter reads 1,600 feet. That means I am too low and I should reduce my vertical speed to slow down my descent. This calculation has to be done by the pilot himself and not by a computer. Furthermore the pilot must fly the plane, i.e. adjust the control surfaces, run landing checklists, visually verify the alignment with the runway, talk to ATC, set the flaps etc. A pilot is trained to do all this, but increased workload also increases the chance of making an error.

  • $\begingroup$ Thank you for your answer. What if you know the height of the runway itself with respect to sea level? Couldn't you use the barometric altimeter reading by subtracting that height from the reading? $\endgroup$ Commented Jun 7, 2017 at 21:08
  • $\begingroup$ @TylerLungrin Yes, you very much can. Indeed, in the UK we actually have another Pressure Setting called the QFE which is used to show you the height above the airfield to save the maths. Most countries just use the local QNH, which is altitude (Above Sea Level) and the pilots can calculate as you say. This isn't accurate enough for Instrument Approaches however $\endgroup$
    – Dan
    Commented Jun 7, 2017 at 21:09
  • 1
    $\begingroup$ @TylerLungrin yes, but this reading is not accurate enough for computers to calculate a vertical guidance. New aircrafts nowadays have the ability to use GPS height when ILS is not available. This trend is spreading, but it is not the majority yet. $\endgroup$
    – kevin
    Commented Jun 7, 2017 at 21:15

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